Crystallization of low silica Na-A and Na-X zeolites from transformation of kaolin and obsidian by alkaline fusion

Cristalizacion de zeolitas Na-A y Na-X bajas en sílice a partir de la transformación de caolín y obsidiana por fusión alcalina

Carlos A. Ríos
Escuela de Geología, Universidad Industrial de Santander, Bucaramanga, Colombia
E-mail: carios@uis.edu.co

Craig D. Williams
School of Applied Sciences, University of Wolverhampton, Wolverhampton, England
E-mail: c.williams@wlv.ac.uk

Oscar M. Castellanos ]]> Programa de Geología, Universidad de Pamplona, Pamplona, Colombia
E-mail: oscarmca@yahoo.es

Eje temático: Materials engineering / Ingeniería de materiales
Recibido: Diciembre 12 de 2011
Aceptado: Noviembre 1 de 2012

Abstract

Zeolites Na-A and Na-X were successfully synthesized from kaolin and obsidian, respectively, following the alkaline fusion method. Raw materials were hydrothermally activated with NaOH to synthesize zeolites Na-A and Na-X. The transformation of the starting materials into zeolites has been evaluated by powder X-ray diffraction, scanning electron microscopy and Fourier transformed infrared spectroscopy to elucidate the crystallization process of zeolites. The use of low cost materials in zeolite synthesis become an area of important interest and research, playing an active role in promoting technological advances, research and technology transfer related to the production of zeolytic materials under well-optimized experimental conditions.

Keywords: Aluminosilicates, Crystallization, Fusion, Hydrothermal, Zeolites.

Resumen

En el presente estudio se llevó a cabo la síntesis de zeolitas Na-A y Na-X a partir de caolín y obsidiana, respectivamente, siguiendo el método de fusión alcalina. Los materiales de partida fueron activados hidrotérmicamente con NaOH para sintetizar productos de alta pureza. La transformación de los materiales de partida en zeolitas ha sido evaluada por técnicas de difracción de rayos X de polvo, microscopia electrónica de barrido y espectroscopia de infrarrojo por transformada de Fourier para dilucidar el proceso de cristalización de zeolitas. El uso de materiales de bajo coste en la síntesis de zeolitas se ha convertido en un área de gran interés e investigación, desempeñando un papel activo en la promoción de los avances tecnológicos, la investigación y transferencia de tecnología relacionados con la producción de materiales zeolíticos en condiciones experimentales controladas.

1. Introduction

Zeolites are abundant and widespread authigenic minerals formed from volcanic glass and various rock-forming minerals by interaction with aqueous solutions in a wide variety of geochemical environments, in particular, as alteration products of aluminosilicates (Mumpton, 1977). Natural zeolites have chemical and structural variations that must be related to the different geochemical environments in which they form (Kawano and Tomita, 1997). To clarify formation conditions of the variations of zeolites, numerous experimental studies on formation of zeolites have been conducted using starting materials such as natural and synthetic glasses, aluminosilicate gels, solution mixture and clay- and rock-forming minerals under a wide range of chemical and temperature conditions (Barrer, 1982).

The preparation of synthetic zeolites from silica and alumina chemical sources is expensive but has the advantage to produce materials of high-purity, with precisely engineered chemical and physical properties suitable for industrial applications and for scientific purposes. Therefore, in order to reduce costs, zeolite researchers are seeking cheaper raw materials for zeolite synthesis, which include clay minerals (Barrer, 1982; Breck, 1974; Boukadir et al., 2002; Klimkiewicz and Drąg, 2004; Ruiz et al., 1997; Baccouche et al., 1998; Cañizares et al., 2000), natural zeolites (Covarrubias et al., 2006), volcanic glasses (Aiello et al., 1982, 1984; Moirou et al., 2000), diatomite (Anderson et al., 2000, 2005), high silica bauxite (Puerto and Benito, 1996), oil shale (Shawabkeh et al., 2004) and natural clinker (Ríos, 2008; Ríos et al., 2008). The bulk chemical composition similarity of the starting materials with the volcanic materials from which the natural zeolites are originated by post-magmatic hydrothermal activity has motivated attempts to synthesize zeolytic materials, justifying the development of future investigations.

The increasing interest in zeolite synthesis from such low-cost materials has promoted the development of various studies on their conversion into zeolytic materials, giving rise to an extensive literature. Zeolite synthesis has been conventionally developed by hydrothermal reaction under alkaline conditions, as reported by several patents and scientific articles. However, the classic alkaline hydrothermal synthesis has been improved by using more sophisticated treatments, which include an alkaline fusion step followed by hydrothermal treatment, the application of microwave-assisted zeolite synthesis and the synthesis of zeolites under molten conditions without any addition of water (Shigemoto et al., 1993; Querol et al., 1997; Park et al., 2000a, 2000b).

Zeolites have been widely used as catalysts, adsorbents and ion-exchangers due to their unique porous structures and special acid-basic properties (Breck, 1974). Their use is becoming increasingly important in several environmental applications and thus is of increasing interest in water purification, particularly for the removal of ammonia, heavy metals, radioactive species and organic substances (Barrer, 1982; Breck, 1974).

Zeolites are crystalline aluminosilicates with open 3D framework structures built of SiO₄ and AlO₄ tetrahedra linked to each other by sharing all the oxygen atoms to form regular intracrystalline cages and channels of molecular

dimensions. A defining feature of zeolites is that their frameworks are made up of 4-coordinated atoms forming tetrahedra. These tetrahedra are linked together by their corners and make a rich variety of beautiful structures, and they are the basic building blocks for various zeolite structures, such as the low-silica zeolites Na-A (LTA, Linde type A) and Na-X (FAU, faujasite), with a molar ratio of Si/Al of 1:1, which are the most common commercial adsorbents. As a consequence they contain the maximum number of cation exchange sites balancing the framework aluminum, and thus the highest cation contents and exchange capacities. Both Na-A and Na-X zeolite frameworks consist of corner sharing TO₄ tetrahedra (T = Si⁴⁺ or Al3⁺).

The TO₄ tetrahedra are often referred to as the primary building units of these zeolite structures. Primary building units are linked together to form secondary building units. The secondary building units consist of n-ring structures (4-ring for Na-A and 6-ring for Na-X), with each corner in the secondary building units representing the center of a tetrahedron. Secondary building units can be linked to form cages or channels within the structure. The aluminosilicate cages and the n-rings connect to form a three dimensional net type structure.

From the perspective of framework structure, both zeolites are stacked up by sodalite (SOD) cages (truncated octahedra) as shown in Figure 1, which can be described with the notation 4668 which means that it is composed of six 4-rings and eight 6-rings. The framework of zeolite Na-A contains two types of cages: β-cages (SOD) and α-cages (supercages of zeolite Na-A); β-cages are linked together by double fourmembered rings (D4R) and form an inner cavity called supercage of zeolite Na-A, which has a diameter of 11.4 Å, is surrounded by 8 β-cages and is interconnected to six other supercages by 6-membered-ring windows. The framework of the zeolite Na-X consists of β-cages (SOD) and α-cages (supercages of zeolite Na-X); β-cages are linked together by double six-membered rings (D6R) and form the supercages of zeolite Na-X, which has a diameter of 13.0 Å, is surrounded by 10 β-cages and is interconnected to four other supercages by tetrahedrally disposed 12-membered-ring windows.

2. Experimental

2.1 Materials and chemical reagents

Both kaolin and obsidian were used as starting materials in zeolite synthesis. Kaolin from South West England is distributed under the name of Supreme Powder and supplied by ECC International Ltd (chemical composition reported by Ríos et al., 2009a, 2009b: 46.44% SiO₂, 38.80% Al₂O₃, 0.03% TiO₂, 0.52% FeO, 0.08% MgO, 0.00% CaO, 0.33% Na₂O and 0.69% K₂O). Obsidian, a volcanic glass, from the Lipari Island (Eolie Archipelago, Italy) has the following chemical composition: 75.27% SiO₂, 12.93% Al₂O₃, 0.08% TiO₂, 1.59% FeO, 0.00% MgO, 0.73% CaO, 4.13% Na₂O and 5.27% K₂O (Nielsen and Sigurdsson, 1981).

The aluminosilicate sources were prepared prior to the synthesis process as follows. Powder preparation techniques included both a SpectroMill Ball Pestle Impact grinder and a mortar and pestle. After grinding, samples were sieved under dry conditions and the size fraction of 200 mesh collected and particles of < 75 μm selected for zeolite synthesis. The dried powder samples were then ground as fine as possible, using a mortar and pestle, for further characterization. Sodium hydroxide (powder) 96% (BDH Laboratory Supplies) of analytical grade and distilled water were used in all experiments.

2.2 Synthesis of zeolites

We adopted in this study an alkaline fusion step prior to the hydrothermal treatment because it plays an important role in enhancing the hydrothermal conditions for zeolite synthesis and because larger amounts of aluminosilicates can be dissolved employing this method. In a typical synthesis process, 6.20 g of the raw material were dry mixed with 7.44 g of NaOH (powder) in a raw material/alkaline activator ratio = 1/1.2 in weight for 30 min and the resultant mixture was fused at 600 °C for 1 h. The alkaline reagent added to the starting material acts as an activator agent during fusion. Some of the inert crystalline phases in the raw materials can be fully reacted. The fused product was ground in a mortar and then 4.40 g of this was dissolved in 21.50 ml of distilled water (ratio = 1/4.9) under stirring conditions to form the amorphous precursors. The amount of reagents used for the preparation of the hydrogels was based on previous literature search (Shigemoto et al., 1993; Ríos, 2008; Ríos et al., 2009a, 2009b).

Hydrogels were aged under static conditions during 24 h. The hydrothermal reaction was carried out under static conditions, transferring the hydrogels to PTFE (polytetrafluoroethylene = Teflon) bottles (Cowie Technology Ltd) of 65 ml for preparations heated at the temperature required of 100 °C for different reaction times (24, 48, 96 h). The reactors were removed from the oven at the scheduled times and were quenched in cold water to stop the reaction. Hydrogel pH was measured before (14.1 for kaolin and 14.2 for obsidian) and after (13.814.0 for kaolin and 13.9-14.1 for obsidian) hydrothermal treatment. Then, the reaction mixtures were filtered and washed with distilled water to remove excess alkali. Finally, the samples were oven dried at 80 °C overnight. The dried samples were weighed and kept in plastic bags for characterization.

2.3 Characterization of the raw materials and as-synthesized zeolites

3. Results and discussion

3.1 Characterization of the starting materials

As shown in the XRD pattern (Figure 2a), kaolinite is the predominant mineral phase, which can be identified by its distinctive reflections at 12.34° and 24.64° of 2θ as reported by Zhao et al. (2004). However, minor impurities, such as illite, muscovite and halloysite, also occur. Kaolinite can be recognized by its platy morphology and hexagonal outlines (Figure 2a), with small well-formed hexagonal plates loosely packed, defining an orientation. XRD pattern of Figure 2b shows the entirely amorphous character of the obsidian, which is a non-hydrated silicate glass that have a three dimensional disordered framework structure. SEM photograph (Figure 2b) of this glass show nearly planar surfaces that have irregularities.

3.2 Characterization of the zeolytic materials

The XRD patterns in Figure 3 show the progress of zeolite synthesis by the fusion method, revealing the reduction of intensity of the characteristic peaks of the starting materials and the appearance of new reflection peaks corresponding to zeolites Na-A and Na-X, which show a progressive increase in the intensity with reaction time. Therefore, the reaction time influences the crystallinity of the synthesis products. Figure 3a reveals a total dissolution of the kaolin-bearing phases. Note the intensities of strong characteristic diffraction peaks for zeolite Na-A, which appeared after 24 h, keeping a constant grade of crystallinity with reaction time. On other hand, Figure 3b shows that after 24 h of reaction a displacement of the hump of the starting material to higher 2-theta values was observed. However, with reaction time a progressive transformation from an amorphous phase to a crystalline phase (zeolite Na-X) occurred after 48 h. The intensities of the reflections peaks of zeolite Na-X increase from 48 to 96 h.

-

The synthesis of zeolites via alkaline fusion followed by hydrothermal treatment reveals that the alkaline fusion process promoted the dry reaction between the mineral phases present in the raw materials and the alkaline activator, and the alkaline fused product corresponds to amorphous sodium aluminosilicate (SEM images), indicating that fusion was very effective in extracting the silicon and aluminium species from kaolin. SEM images of the morphology of the as-synthesized zeolytic products obtained after activation of the starting materials are exhibited in Figure 4. Figure 4(a-b) shows the typical morphology of well-developed cubes of zeolite Na-A displaying fluorite-type interpenetration-twinning, similar to those reported in previous studies (Gerson and Zheng, 1997; Porcher et al., 2000; Zhao et al., 2004).

The presence of aluminosilicate framework in the zeolites was confirmed using the FTIR spectroscopic technique and the results are shown in Figure 5. The internal tetrahedral bending peak (~ 464 cm⁻¹) for kaolin does not exists in the synthesis products shifts from 456 to 459 cm⁻¹ due to the incorporation of alumina tetrahedron into the zeolite framework. The vibration band at 433 cm⁻¹ for obsidian changed to 411 (amorphous material) and 439449 (zeolite Na-X) cm⁻¹. The emergence of crystal phase is characterized by the double ring external linkage peak (543-545, 552 and 599 cm⁻¹), internal linkage symmetrical stretching peak (652-658 and 660-677 cm⁻¹), external tetrahedral symmetrical stretching peak (733 cm⁻¹) and internal tetrahedral asymmetrical stretching peak (966-981 and 973-986 cm⁻¹). Observe how the peaks change suddenly from 24 to 48 h using obsidian, taking into account that after 24 h the synthesis product corresponds to an amorphous material, whereas after 48 h zeolite Na-X appears. During hydrothermal reaction, the characteristic vibration bands of the raw materials disappeared, accompanied by zeolite crystal grow rapidly. It means that the formation of the zeolite crystal happens very quickly in the synthesis solution. The sixmembered rings, as secondary building unit of the zeolite, are formed during the induction period, while the double ring structure, tertiary building unit of these zeolites, is formed afterwards (Hu et al., 2009). All these observations confirm the formation of zeolites Na-A and Na-X after alkali fusion followed by hydrothermal treatments of the raw materials. The different intensity of the vibration bands of the synthesis products confirms XRD patterns concern with the grade of crystallinity.

3.3 Reaction mechanism during zeolite synthesis

The reaction mechanism of zeolite synthesis from kaolin and obsidian via alkaline fusion prior to hydrothermal reaction is discussed here and can be divided into a number of steps: fusion of the starting material with NaOH, dissolution of the alkaline fused product releasing Si and Al (preparation of the reaction gel) and hydrothermal reaction at 80 ºC for several reaction times (24, 48 and 96 h).

The chemistry of evolution of the system, such as zeolites Na-A and Na-X, is very complex due to several factors, such as impurities present in the starting materials, gel composition and pH, aging conditions, reaction temperature and time, formation of intermediate metastable phases, and nucleation and growth of more stable phases, among others. However, the reaction history of the zeolite synthesis can be summarized as follows. A simple scheme of the crystallization of an amorphous aluminosilicate hydrogel to zeolites Na-A and Na-X is given in Figure 6.

The alkaline fusion (solid-state reaction) step at 600 °C (Figure 6a) enhances the dissolution of the Al₂O₃ and SiO₂ from the starting materials, mostly converting them into sodium salts (Na₂SiO₃ + Na₂AlO₂)

The step of gel preparation (Figure 6b), NaSiO + NaAlOare dissolved in HO at 2322 2room temperature to produce the reaction gel with a chemical composition, which can be expressed as [Na(AlO)(SiO).NaOH.HO] x2y2z 2(gel) as suggested by Kondru et al. (2011).

During the formation of zeolites Na-A and Na-X in the hydrothermal reaction, the following events occur: induction period, which is followed by nucleation and crystal growth. For the induction period, reaction intermediates remain mostly amorphous as a gel, prior to crystallization. According to Epping and Chmelka (2006), with the onset of crystallization, most of the intermediate gel transforms relatively rapidly to crystalline zeolytic product. This process appears to occur more rapidly in more concentrated gels, in which a greater number of small crystalline nuclei are expected to form during the induction period (Shi et al., 1996). During the induction period, a reorganization of the amorphous aluminosilicate gel occurred. It can lead to the formation of "crystalline" zeolite nanoparticles throughout the amorphous gel (Epping and Chmelka, 2006).

A polymerization (Figure 6d) should be the process that forms the zeolytic precursors, which contains tetrahedra of Si and Al randomly distributed along the polymeric chains that are cross-linked so as to provide cavities sufficiently large to accommodate the charge balancing alkali ions. It is well known that zeolites are based on the primary tetrahedral building unit (TO₄) where the central tetrahedrally bonded (T) atom is usually either a silicon or aluminium atom, surrounded by four oxygen atoms. By linking these tetrahedra together, it is possible to build larger structures, as shown in Figure 6d. According to Itani et al. (2009), during the initial polymerization of aluminosilicate species a significant part of the Na⁺ is expelled from the gel into the solution, which restricts extensive polymerization and leads to formation of small aluminosilicate particles with open pore structure.

During nucleation, the gel composition was significantly affected by thermodynamic and kinetic parameters (Ojha et al., 2004). Gel composition and structure are significantly affected by thermodynamic and kinetic parameters. According to Breck (1974), they are controlled by the size and structure of the polymerization species and gelation controls the nucleation of the zeolite crystallites. When the TO₄ units are linked together to form a zeolite framework, they characteristically yield periodic cavities and channels throughout the structure (Figure 6e).

The primary TO₄ units can be linked to create secondary building units (SBU's). In zeolites Na-A and Na-X, a combination of 4- and 6-rings promoted the formation of the β-cage. Crystallization generally involved the assimilation of material from solution by a crystal growth process, which began when the nuclei reached a critical size and the crystals started to grow. A stable phase reached the required equilibrium conditions to promote the crystal growth of zeolites Na-A and Na-X. The lineal growth rates of zeolites show a strong dependence with temperature. However, these rates are also influenced by several variables, particularly the molar composition of the reaction gel. The hydrothermal treatment of the reaction gel at the scheduled temperature and time of reaction produced the crystallization of zeolites Na-A and Na-X. The Na⁺ cation also plays a very important role during zeolitization, because it stabilizes the sub-building units of zeolite frameworks and is fundamental for zeolite synthesis under hydrothermal conditions (Fernández-Jiménez et al., 2005). We are in agreement with Smaihi et al. (2003) that the first crystallization stage proceeds by reorganization of the amorphous aluminosilicate units formed during the mixing of the precursors. The crystallization process takes place in the volume of the aggregates by propagation through the gel phase (Smaihi et al., 2003). This propagation through the gel continues to complete the transformation of the particles into zeolites Na-A and Na-X. After this stage, the growth in the system is dominated by the solution-mediated transport, which leads to the crystallization of well-shaped crystals of zeolites Na-A and Na-X and consequently their structures (Figure 6f).

4. Potential environmental applications of the synthesized zeolytic materials

Major prospective applications of the synthesized zeolytic materials are based on their use as high ion exchangers in industrial wastewater and soil decontamination. Previos studies by Ríos and co-workers (Ríos, 2008; Ríos et al., 2008; Contreras and Castrillon, 2011) reveal that the use of zeolites Na-A and Na-X may be an alternative to heavy metal removal from industrial wastewater. In general, the results are very encouraging about the future industrial applications of the synthesized zeolytic materials. The further investigation of the synthesized zeolytic materials, which will be conducted in the near future, includes the test of their ability of precipitating or adsorbing heavy metals from industrial wastewaters.

5. Conclusions

Both kaolin and obsidian are suitable as raw materials for the synthesis of pure zeolites Na-A and Na-X in the system of SiO₂-Al₂O₃-Na₂OH₂O using NaOH as activating agent and an alkaline fusion step followed by hydrothermal treatment. By fusion with NaOH powder, mineral phases in the starting materials can be completely changed into a Na aluminosilicate. The synthesis method introducing an alkaline fusion step prior to the hydrothermal treatment produced crystalline zeolites after short times of crystallization. The results obtained in this study prove that further studies should be carried out under well-optimized experimental conditions to successfully prepare highly crystalline zeolites with potential application in the purification of industrial wastewater.

6. Acknowledgments

This research was supported by the Programme Alban, "the European Union Programme of High Level Scholarships for Latin America" (Scholarship No. E05D060429CO) and the Universidad Industrial de Santander, and has benefited from research facilities provided by the School of Applied Sciences at the University of Wolverhampton. The authors are indebted to Dr. David Townrow, Mrs Diane Spencer, and Mrs. Barbara Hodson for their technical assistance with the acquisition of XRD, FTIR and SEM data. We are most grateful to the above-named people and institutions for support.

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